Nouns ending in a in the singular and e in the plural, e.g. "la ragazza" / "le ragazze": most nouns in this class are feminine.
Nouns ending in o in the singular and i in the plural, e.g. "il ragazzo" / "i ragazzi": most nouns in this class are masculine.
Nouns ending in e in the singular and i in the plural, e.g. "il pesce" / "i pesci": nouns in this class can be any gender.
Nouns ending in a in the singular and i in the plural, e.g. "il problema" / "i problemi": most nouns in this class are masculine.
While it can be said that Italian nouns ending in "a" are feminine and those ending in "o" are masculine, not all nouns end in "a" or "o". Unfortunately for those of us learning the language, the nouns that end in "e" in the singular can be either masculine (il giornale) or feminine (la tigre).
Coming back to this after learning more since I wrote it: Feminine nouns GENERALLY end with a, male with O, and lots of words ending with "e" are masculine, but it's not a hard rule. There are exceptions: feminine ending with O (Metro), and in italian, "tigre" is feminine...just the way it is.